The invention relates to a rotor slot insulation system for an electrical machine and to an article incorporating same and, more particularly, to a rotor slot insulation system wherein the insulation is inhibited from undesirable axial movement while being protected from potential compromise of the insulative integrity at the bottom corners of the slot due to concentrated load applied at the corners by the edge of a conductor to be disposed within the slot. This invention may be beneficially employed in any electrical machine having a rotor in which slots are disposed for receiving an electrical conductor and wherein the electrical conductor must be insulated from the material constituting the body of the rotor. It may be especially beneficially employed in an electric generator, or dynamoelectric machine having a rating greater than about 3000 KW, wherein the electrical conductor lying in the axial extending slot of the rotor body constitutes a field winding and further wherein the rotor is typically centrally disposed within a stationary circumferentially surrounding stator or armature winding.
Ground insulation, or slot armor, which is disposed in coil slots of a rotor for insulating conductors to be disposed in the slots from the body of the rotor, must be prevented from axially migrating out an end of the rotor slot. Such migration may create one or more grounds on the conductor wherein subsequent current flow through the ground path may damage the rotor body and results in a reduction in efficiency. Such migration may also create a thermal inbalance and produce undesirable rotor vibration. Such migration is caused, in part, by thermal cycling in response to changes in current necessary to accommodate varying output power demanded from, or input to the machine.
Historically, ground insulation coil slots was made from a relatively compliant material formed into a U-shaped channel into which coils were wound with essentially zero side clearances. After the coil was wound, the two radially protruding sides of the insulation were folded over. Any remaining space in the slot between the top of the coil and a rotor wedge was packed with a filler material. During operation, outward centrifugal force exerted by the coil conductors against the rotor wedges was sufficient to secure the ground insulation against axial motion. Thus no creepage or spacer strip was required at the bottom of the slot for securing the ground insulation against undesirable axial motion within the slot.
The relatively compliant material that was formed into a U-shaped channel has subsequently been replaced in many applications by materials having superior mechanical qualities. However, these latter materials are generally less pliable and are therefore typically pre-formed as L-shaped pieces. Opposing horizontal legs (typically the shorter legs) of L-shaped pieces are butted together at the bottom of the coil slot to form the substantially U-shaped insulation of the earlier designs. However, this butt joint creates a very short electrical creepage path to ground potential across the thickness of the insulation and thus it is necessary to provide a larger creepage path, such as by inserting a strip of electrically insulative material, known as a creepage strip, along the bottom (radially innermost position) of the coil slot between the rotor body and the horizontal legs of the L-shaped pieces.
In one configuration which has been used to secure the L-shaped insulation within the slot, the L-shaped insulation is extended beyond an axial end of the rotor body and wedges are applied between adjacent slots to force the vertical leg (typically the longer leg) of one L-shaped member of each adjacent slot into the side of the conductors within the respective slot of the leg, so that frictional forces secure the respective L-shaped pieces against axial movement. However, in a certain classes of rotors, a cooling channel axially extends between adjacent slots from the axial end of the rotor body. In such cases, the block for wedging the L-shaped insulation may interfere with coolant fluid flow into the cooling channel. Wedges having an opening through them have been used to allow cooling fluid to pass therethrough and onto the cooling channel. Such wedges cannot be fitted as well, and do not provide the same level of restraint against axial movement of the slot insulation, as do solid wedges. Besides securing the L-shaped insulation pieces within the slot, it is also necessary to secure the creepage strip against undesirable axial motion, which has been accomplished by fixedly attaching a radially inwardly extending button to the creepage strip and mating it with a cooperating recess, fabricated, such as by drilling, into the bottom of the coil slot.
In addition, in order to be accommodated within the coil slot without compromising the insulative quality of the L-shaped pieces, the radially innermost conductor in the slot must have generously radiused edges to coincide with the inside radius of the junction of the vertical and horizontal legs of the L-shaped insulation. Without this radius, the relatively sharp, squared-off edges of the radially innermost conductor exert a concentrated load against the slot insulation, which may cause it to crack, thereby compromising the insulative integrity of the L-shaped insulation. However, from an efficiency standpoint, it is generally desirable to provide the maximum amount of conductor material, e.g. copper, and therefore radiusing edges of a typically rectangular conductor should be avoided. Also, radiusing requires additional fabrication steps, which necessitate additional handling and expense.
Accordingly, it is an object of the present invention to secure rotor slot insulation against undesirable axial movement without intefering with coolant gas flow.
Another object of the present invention is to prevent compromising the insulative integrity of slot insulation from unradiused conductors lying within the slot.
Yet another object of the present invention is to eliminate fabrication and machining steps required to provide recesses in the bottom of the rotor slot for receiving buttons for securing the creepage strip.
Still another object of the present invention is to eliminate fabrication of radiused edges in conductors to be disposed in the slot.